Added section explaining why is async mode recommended (#444)

Author: lcrespom

README.md

@@ -142,6 +142,9 @@ bcrypt.compareSync(myPlaintextPassword, hash); // true
 bcrypt.compareSync(someOtherPlaintextPassword, hash); // false
 ```
 
+### Why is async mode recommended over sync mode?
+If you are using bcrypt on a simple script, using the sync mode is perfectly fine. However, if you are using bcrypt on a server, the async mode is recommended. This is because the hashing done by bcrypt is CPU intensive, so the sync version will block the event loop and prevent your application from servicing any other inbound requests or events.
+
 ## API
 
 `BCrypt.`